Microtubules are one of the components of the cytoskeleton. They have diameter of ~ 24 nm and length varying from several micrometers to possibly millimeters in axons of nerve cells. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular transport. From [1], believed to be in the public domain File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... From [1], believed to be in the public domain File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The eukaryotic cytoskeleton. ... A nanometre (American spelling: nanometer, symbol nm) is a unit of length in the metric system, equal to one thousand-millionth of a metre, which is the current SI base unit of length. ... Mitosis is the process in which a cell duplicates its chromosomes to generate two identical cells. ... A cell that has almost completed cytokinesis. ... In cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. ...

Structure

Microtubules are polymers of α- and β-tubulin dimers. The tubulin dimers polymerize end to end in protofilaments. The protofilaments then bundle in hollow cylindrical filaments. Typically, the protofilaments arrange themselves in an imperfect helix with one turn of the helix containing 13 tubulin dimers each from a different protofilament. The image above illustrates a small section of microtubule, a few αβ dimers in length. Tubulin is the protein which makes up microtubules. ... Sucrose, or common table sugar, is composed of glucose and fructose. ...

Another important feature of microtubule structure is polarity. Tubulin polymerizes end to end with the α subunit of one tubulin dimer contacting the β subunit of the next. Therefore, in a protofilament, one end will have the α subunit exposed while the other end will have the β subunit exposed. These ends are designated (−) and (+) respectively. The protofilaments bundle parallel to one another, so in a microtubule, there is one end, the (+) end, with only β subunits exposed while the other end, the (−) end, only has α subunits exposed. A commonly-used example of a polar compound is water (H2O). ... A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ...

Organization within cells

Microtubules are nucleated and organized by the microtubule organizing centers (MTOCs), such as centrosomes and basal bodies. They are part of a structural network (the cytoskeleton) within the cell's cytoplasm, but, in addition to structural support, microtubules take part in many other processes, as well. They are capable of growing and shrinking in order to generate force, and there are also motor proteins that move along the microtubule. A notable structure involving microtubules is the mitotic spindle used by eukaryotic cells to segregate their chromosomes correctly during cell division. Microtubules are also part of the cilia and flagella of eukaryotic cells (prokaryote flagella are entirely different). Microtubules also move organelles and cell structures to new locations. MTOC or microtubule-organizing center is a structure found in all plant and animal cells from which microtubules radiate. ... The centrosome is the main microtubule organizing center (MTOC) of the cell as well as a regulator of cell-cycle progression. ... A basal body is a short cylindrical array of microtubules plus their associated proteins found at the base of a eukaryotic cell cilium or flagellum. ... The eukaryotic cytoskeleton. ... It has been suggested that Cytoplast be merged into this article or section. ... Micrograph showing condensed chromosomes in blue and the mitotic spindle in green during prometaphase of mitosis The mitotic spindle (a. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista Alternative phylogeny Unikonta Opisthokonta Amoebozoa Bikonta Apusozoa Cabozoa Rhizaria Excavata Corticata Archaeplastida Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. ... cross-section of two cilia, showing 9+2 structure A cilium (plural cilia) is a fine projection from a eukaryotic cell that constantly beats in one direction. ... // A Flagellum (plural: flagella) is a long, slender projection from the cell body, composed of microtubules and surrounded by the plasma membrane. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista Alternative phylogeny Unikonta Opisthokonta Amoebozoa Bikonta Apusozoa Cabozoa Rhizaria Excavata Corticata Archaeplastida Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. ... Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. ...

Nucleation and growth

Polymerization of microtubules is nucleated in a microtubule organizing center. Contained within the MTOC is another type of tubulin, γ-tubulin, which is distinct from the α and β subunits which compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a circular structure known as the "γ-tubulin ring complex." This complex acts as a scaffold for α/β tubulin dimers to begin polymerization; it acts as a cap of the (−) end while microtubule growth continues away from the MTOC in the (+) direction.

Dynamic instability

During polymerization, both the α- and β-subunits of the tubulin dimer are bound to a molecule of GTP. The GTP bound to α-tubulin is stable, but the GTP bound to β-tubulin may be hydrolized to GDP shortly after assembly. The kinetics of GDP-tubulin are different from those of GTP-tubulin; GDP-tubulin is prone to depolymerization. A GDP-bound tubulin subunit at the tip of a microtubule will fall off, though a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out. Since tubulin adds onto the end of the microtubule only in the GTP-bound state, there is generally a cap of GTP-bound tubulin at the tip of the microtubule, protecting it from disassembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe. GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as rescue. Guanosine triphosphate (GTP) is also known as guanosine-5-triphosphate. ...

Image File history File links Download high resolution version (933x509, 19 KB)Microtubule dynamic instability Copyright 26 September 2004 by Nicholas Ingolia File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

In vivo microtubule dynamics vary considerably. Assembly, disassembly and catastrophe rates depend on which microtubule-associated proteins (MAPs) are present. In cell biology, microtubule-associated proteins (MAPs) are proteins that interact with the microtubules of the cellular cytoskeleton. ... In cell biology, microtubule-associated proteins (MAPs) are proteins that interact with the microtubules of the cellular cytoskeleton. ...

Chemical effects on microtubule dynamics

Microtubule dynamics can also be altered by drugs. For example, the taxane drug class (e.g. paclitaxel or docetaxel), used in the treatment of cancer, blocks dynamic instability by stabilizing GDP-bound tubulin in the microtubule. Thus, even when hydrolysis of GTP reaches the tip of the microtubule, there is no depolymerization and the microtubule does not shrink back. Nocodazole and Colchicine have the opposite effect, blocking the polymerization of tubulin into microtubules. The taxanes are NOT alkaloids (not every molecule coming from a plant and bearing a nitrogen is an alkaloid!). They are diterpenes produced by the plants of the genus Taxus (yews). ... Paclitaxel is a drug used in the treatment of cancer. ... Docetaxel chemical structure Docetaxel is a chemotherapy drug used in the treatment of cancer. ... Nocodazole is an anti-neoplastic agent which exerts its effect by depolymerizing microtubules. ... Colchicine is a highly deadly poisonous alkaloid, originally extracted from plants of the genus Colchicum (Autumn crocus, also known as the Meadow saffron). Originally used to treat rheumatic complaints and especially gout, it was also prescribed for its cathartic and emetic effects. ...

Motor proteins

In addition to movement generated by the dynamic instability of the microtubule itself, the fibers are substrates along which motor proteins can move. The major microtubule motor proteins are kinesin, which generally moves towards the (+) end of the microtubule, and dynein, which generally moves towards the (−) end. The kinesin dimer attaches to, and moves along, microtubules. ... Dynein is a motor protein (also called molecular motor or motor molecule) in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. ...

Microtubules and consciousness

Microtubules have been ascribed a key role in explaining the features of consciousness in a controversial and highly speculative theory developed by Roger Penrose and Stuart Hameroff, known as Orch-OR (orchestrated objective reduction). This theory states that consciousness is a result of quantum computing in the brain, based on tubulin molecules in microtubules as qubits.The theory is regarded with considerable scepticism, and has recently been subjected to a quantitative critique by Max Tegmark, which argued that the observed time scale of neuron firing and excitations in microtubules is slower than the timescales predicted by the theory by a factor of at least 10,000,000,000. Image File history File links Unbalanced_scales. ... Consciousness is a quality of the mind generally regarded to comprise qualities such as subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and ones environment. ... Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the Mathematical Institute, University of Oxford and Emeritus Fellow of Wadham College. ... Stuart Hameroff, MD, is an anesthesiologist and professor at the University of Arizona known for his promotion of the scientific study of consciousness, and his speculative theories of the mechanisms of consciousness. ... Orch OR (“Orchestrated Objective Reduction”) is a theory of consciousness put forth in the mid-1990s by British theoretical physicist Sir Roger Penrose and American anesthesiologist Stuart Hameroff. ... Molecule of alanine used in NMR implementation of error correction. ... Tubulin is the protein which makes up microtubules. ... A qubit is not to be confused with a cubit, which is an ancient measure of length. ... Max Tegmark Max Tegmark born 1967 in Sweden to Karin Tegmark and Harold S Shapiro, is a cosmologist formerly at the University of Pennsylvania and now at the Massachusetts Institute of Technology as an Associate Professor. ...

In addition to the problems of time scale and size, Orch-OR relies on the ability of tubulin to exist in one of two equally likely states. While tubulin does have two different conformations (a GDP-bound curved conformation and a GTP-bound straight conformation), the GDP-bound state is the only state that is found within the interior of a microtubule. GTP-bound tubulin is only present in the cell at the very end of a polymerizing microtubule and free in the cytoplasm as isolated tubulin dimers.

Additional images

Proteins in different cellular compartments and structures tagged with green fluorescent protein. Image File history File linksMetadata Download high-resolution version (469x618, 71 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Protein Gfp-cdna Metadata This file contains additional information, probably added from the digital camera or... Cellular compartments in cell biology comprise all closed parts within a cell whose lumen is usually surrounded by a single or double lipid layer membrane. ... It has been suggested that mGFP be merged into this article or section. ...